With monetary policy largely on hold for a while, Bank of Japan officials may devote some portion of next week’s board meeting to discussing fiscal policy — in particular, how to gently prod Prime Minister Shinzo Abe to stick with a planned sales tax increase, despite growing opposition from some of his advisers.

It won’t come up in the official statement released Thursday at the end of the two-day session, people familiar with the central bank’s thinking told The Wall Street Journal. But policy board members may raise it with the government representatives attending those meetings — and, thus, have the discussion made public in the minutes to be published Sept. 10, about the time a final decision needs to be made. Gov. Haruhiko Kuroda is also prepared to advocate for the tax at his press conference Thursday afternoon, following up on supportive comments he made in a speech earlier this week.

While some Abe advisers fear that raising the sales tax to 8% from 5% in April could damp consumption and growth, BOJ officials fear any delay could hurt the economy by leading to a spike in interest rates, as investors lose faith in Japan’s commitment to paying down its debt, and begin to demand higher returns to compensate for the risk of holding Japanese government bonds. Borrowing costs would then increase across the board, discouraging consumers and businesses from borrowing and spending.

“That would undermine the effects of the (BOJ’s current) quantitative and qualitative easing,” which among other things aims to spur growth through lower interest rates, one of these people said.

Some BOJ officials now see the need to “mentally run simulations” over what could happen in case the government reopens the tax law, according to another person familiar with the central bank’s thinking.

Since Japan’s parliament last summer passed a law to double the sales tax in two stages between April 2014 and October 2015, the BOJ has assumed that the government will carry it out to help slow exponential growth in its public debt, now close to 250% of annual economic output.

But in recent weeks, senior government officials have begun sending confusing signals. Finance Minister Taro Aso strongly backs that tax policy, but key advisors to Prime Minister Abe are calling for slowing the pace of tax hikes to reduce their impact on growth. Mr. Abe has been non-committal.

While the BOJ still believes that the main scenario is for the government to respect the current tax schedule, the bank is nevertheless starting to fret over possible delays or changes to the tax plan. Thus, the move by some BOJ officials to consider gently reminding the government of the importance of sticking to the tax schedule at the bank’s planned policy board meeting next Wednesday and Thursday. Finance ministry and Cabinet office officials regularly attend BOJ board meetings as observers.

The bank’s worries underline the fact that even though Japan’s economic recovery is gaining speed, it could get off the track if the country’s fiscal problems implode.

BOJ officials fear that investors might at some point might start to suspect that the central bank was merely printing money to fund government expenditures, or “monetizing” debt, those people said. The BOJ is indeed vulnerable to such speculation. Under its aggressive monetary easing program launched in April, the bank buys JGBs amounting to a whopping 70% of newly issued debt. In that sense, fiscal overhaul is now “essential” for the BOJ program because, otherwise, speculation over monetization could emerge, one person said.

Any backtracking on fiscal overhaul would also be a slap in the BOJ’s face.

In January, the BOJ signed a joint policy statement with the government vowing to generate 2% inflation as soon as possible in part because the government promised to “steadily promote measures aimed at establishing a sustainable fiscal structure,” people familiar with the matter said. Back then the BOJ indeed “insisted” that the government commit to fiscal overhaul so there would be no interest rate jumps or a Europe-style fiscal crisis, one of them said.

The BOJ’s subtle step into fiscal policy — usually left to elected officials — follows Mr. Abe’s unusual heavy-handed intrusion earlier this year into monetary policy, usually left to the central bank. It was Mr. Abe who demanded the BOJ set an inflation target, and chose Mr. Kuroda specifically to engineer a “regime change” at the BOJ.

But the BOJ intends a more understated move into Mr. Abe’s turf. There is a traditional “culture” within the central bank to try to avoid openly butting heads with the government, so any message its board may send to the government next week would probably be nothing combative, the people said.

About Japan Real Time

Japan Real Time is a newsy, concise guide to what works, what doesn’t and why in the one-time poster child for Asian development, as it struggles to keep pace with faster-growing neighbors while competing with Europe for Michelin-rated restaurants. Drawing on the expertise of The Wall Street Journal and Dow Jones Newswires, the site provides an inside track on business, politics and lifestyle in Japan as it comes to terms with being overtaken by China as the world’s second-biggest economy. You can contact the editors at japanrealtime@wsj.com